A green LED

Abstract

The present application relates to a kind of green light LED, including n-type layer, multiple quantum well layer, superlattice electron blocking layer, p-type layer and p-type electrode which are sequentially stacked on substrate;Multiple quantum well layer includes n periodic quantum well structure, single periodic quantum well structure is sequentially stacked by InGaN layer, first GaN layer, AlGaN interlayer and second GaN layer, and the Al component of AlGaN interlayer increases layer by layer in the direction of substrate pointing to p-type layer;Superlattice electron blocking layer is composed of m periodic AlGaN / GaN stack and AlGaN thick layer arranged on the stack;The setting of the multiple quantum well layer and superlattice electron blocking layer effectively improves the barrier height, reduces the electron leakage, relieves the moving speed of hot electron, promotes the faster diffusion of carrier, improves current congestion effect, has more optimal radiation performance and higher modulation bandwidth.

Description

Technical Field

[0001] The present invention relates to the technical field of optical communications, and in particular to a MINI green light LED with high modulation bandwidth and high light output efficiency. Background Art

[0002] The emergence of LED optical communication technology has alleviated the shortage of RF communication spectrum. Optical communication has rapidly become a hot topic of research due to its advantages of high speed, wide bandwidth, low loss, and low interference. Mini LEDs, which are LED devices smaller than 200μm, have superior current spreading compared to larger LEDs. As LED size decreases, the lateral propagation of the LED beam decreases, resulting in higher luminous efficiency. Furthermore, the reduced horizontal resistance improves current distribution, significantly enhancing the current spreading effect and carrier recombination efficiency, further increasing bandwidth. Mini LEDs' high modulation bandwidth and high current density make them a popular choice as signal sources for LED visible light communication. However, the strong polarization caused by the increased mismatch stress between AlGaN and GaN leads to a severe quantum confined Stark effect (QCSE), exacerbating electron leakage and non-radiative recombination, particularly Auger recombination. Due to this non-radiative recombination, the optical output power (LOP) of current MINI LEDs is very low, below 1mW, and the wall-mount efficiency (WPE) is less than 1%, making them unable to meet long transmission distance requirements. Both modulation bandwidth and optical output efficiency are limited, making them difficult to meet high-speed communication requirements. Summary of the Invention

[0003] In response to the technical problems existing in the prior art, the primary purpose of the present invention is to provide a green light LED with high modulation bandwidth and high light output efficiency. The carrier concentration distribution in the active region is more uniform, which accelerates the diffusion movement of carriers and greatly increases the probability of carrier radiative recombination, thereby reducing the lifetime of radiative recombination and further improving the light output efficiency and modulation bandwidth.

[0004] In one aspect, the present invention provides a green light LED, comprising an n-type layer, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer, and a p-type electrode stacked sequentially on a substrate;

[0005] The multi-quantum well layer includes a quantum well structure with n periods, wherein a single period of the quantum well structure is formed by sequentially stacking an InGaN layer, a first GaN layer, an AlGaN insertion layer, and a second GaN layer. In the multi-quantum well layer, the Al component of the AlGaN insertion layer increases layer by layer in a direction from the substrate to the p-type layer, with 2≤n≤5. The provision of the AlGaN insertion layer increases the effective barrier height, reduces electron leakage, effectively alleviates the movement speed of hot electrons, promotes faster diffusion of carriers, and improves current congestion.

[0006] The superlattice electron blocking layer is composed of an AlGaN / GaN stack of m periods and an AlGaN thick layer arranged on the stack, 2≤m≤5; the setting of the superlattice electron blocking layer ultimately increases the effective barrier height of electrons to the maximum, reduces the effective barrier height of holes, improves the injection efficiency of carriers, and improves the radiation luminescence performance.

[0007] Furthermore, in the multi-quantum well layer, the Al composition of the AlGaN insertion layer in the quantum well structure of the first cycle is 0.04, the Al composition of the AlGaN insertion layer in the quantum structure of the last cycle is 0.2, and the difference in the Al composition of the AlGaN insertion layer in the quantum well structure of adjacent cycles is constant.

[0008] Furthermore, in a single-period quantum well structure, the thickness of the InGaN layer is 3 nm, the thickness of the first GaN layer is 3 nm, the thickness of the AlGaN insertion layer is 1 to 2 nm, and the thickness of the second GaN layer is 3 nm.

[0009] Furthermore, in the AlGaN / GaN stack, the thickness of the AlGaN layer is 1-2 nm, and the thickness of the GaN layer is 1 nm.

[0010] Furthermore, in the AlGaN / GaN stack, the Al composition of the AlGaN layer is 0.2.

[0011] Furthermore, the thickness of the AlGaN thick layer is 3-6 nm, and the Al composition of the AlGaN thick layer is 0.2.

[0012] Furthermore, the p-type layer is a p-type GaN layer.

[0013] Furthermore, the n-type layer is an n-type GaN layer, and an n-type electrode is further provided on the n-type GaN layer.

[0014] Furthermore, the substrate is a sapphire or silicon carbide substrate.

[0015] Furthermore, the green LED is a MINI green LED.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects:

[0017] The present invention forms a single-period structure of InGaN / GaN / AlGaN / GaN by inserting an AlGaN insertion layer into the GaN barrier layer of a multi-quantum well structure. At the same time, the Al component of the AlGaN insertion layer increases layer by layer as the period increases. A superlattice electron blocking layer consisting of an AlGaN / GaN stacking period and an AlGaN thick layer disposed on the stacking period is provided on the multi-quantum well structure. This increases the effective barrier height, reduces electron leakage, effectively alleviates the mobility of hot electrons, promotes faster diffusion of carriers, improves current congestion, and ultimately increases the effective barrier height of electrons to a maximum while reducing the effective barrier height of holes. In one embodiment, the effective barrier height of electrons is increased to 462.4 meV, and the effective barrier height of holes is reduced to 338.2 meV, both of which are superior to conventional structures and inserted barrier structures with unchanged Al component. Both the electron concentration and the hole concentration in each quantum well are increased.

[0018] The present invention proposes inserting an AlGaN insertion layer with gradually increasing composition into the GaN barrier layer of the multi-quantum well structure, and maximizing the carrier injection efficiency in combination with the AlGaN / GaN stacked periodic structure, with better radiative luminescence performance and higher modulation bandwidth. In one embodiment, at 40mA, the light output power reaches 222.93mW, and the efficiency decreases from 87% to 48.6%. The more composite carriers there are inside the active region, the greater the probability of radiative recombination of the carriers, the shorter the lifetime of radiative recombination, and the corresponding device has a higher 3dB modulation bandwidth. In one embodiment, the modulation bandwidth of the green light LED is increased from 76.9MHz to 93.8MHz at a turn-on voltage of 3.86V and 60mA. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 FIG. 1 is a schematic structural diagram of a green LED according to an embodiment of the present invention.

[0020] Figure 2 FIG. 1 is a schematic diagram of the energy band between the last QB and EBL in a green LED structure according to an embodiment of the present invention.

[0021] Figure 3 Schematic diagram of carrier movement among the n-GaN layer, the multi-quantum well MQWs layer, the SL EBL layer and the p-GaN layer in the green LED structure according to an embodiment of the present invention.

[0022] Figure 4 Schematic diagram of the structure of the MQW layer in a proportional green LED structure of the present invention.

[0023] Figure 5 Schematic diagram of the structure of the MQW layer in a proportional green LED structure of the present invention.

[0024] Figure 6 Schematic diagram of the structure of the MQW layer in a proportional green LED structure of the present invention.

[0025] Figure 7 FIG. 1 is a schematic structural diagram of an MQW combined with an SL EBL layer in a green LED structure according to an embodiment of the present invention.

[0026] Figure 8 This is a diagram of the light output power of LED D, LED A, LED B, and LED C of the present invention at different currents.

[0027] Figure 9 Graphs of the internal quantum efficiency of LED D, LED A, LED B, and LED C of the present invention at different currents.

[0028] Figure 10 This is a transient response curve diagram of LED D, LED A, LED B, and LED C of the present invention under a 60mA square wave current pulse. DETAILED DESCRIPTION

[0029] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention, combined with the accompanying drawings. The described embodiments are only a portion of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments derived by persons of ordinary skill in the art without inventive effort are also within the scope of protection of the present invention.

[0030] Spatially relative terms such as "below," "beneath," "below," "above," "upper," etc. are used in this specification to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures.

[0031] In addition, the use of terms such as "first," "second," and the like to describe various elements, layers, regions, sections, and the like is not intended to be limiting. The use of "having," "containing," "including," and "comprising" are open-ended terms that indicate the presence of stated elements or features, but do not exclude additional elements or features, unless the context clearly indicates otherwise.

[0032] The embodiment of the present invention provides a MINI green LED with high modulation bandwidth and high light output efficiency, and its structure is shown in FIG. Figure 1 The green LED comprises a substrate 1, an n-type layer 2, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer 14 and a p-type electrode 15 stacked in sequence on the substrate 1.

[0033] The substrate 1 is a sapphire substrate or a silicon carbide substrate. The n-type layer 2 is a Si-doped n-type GaN layer. In a preferred embodiment, its doping concentration is 5×10 18 cm -3 , with a thickness of 4μm;. The multi-quantum well layer includes n periods of quantum well structure, 2≤n≤5, and a single period of quantum well structure is composed of an InGaN layer, a first GaN layer, an AlGaN insertion layer, and a second GaN layer stacked in sequence; along the direction from the substrate to the p-type layer, the Al composition of the AlGaN insertion layer increases layer by layer. In the first period of quantum well structure (i.e., the period adjacent to the n-type GaN), the Al composition of the AlGaN insertion layer is 0.04, and in the last period of quantum well structure (i.e., the period adjacent to the superlattice electron blocking layer), the Al composition of the AlGaN insertion layer is 0.2. The difference in the Al composition of the AlGaN insertion layer in the quantum well structure of adjacent periods is constant; in a single quantum well period, the thickness of the InGaN layer is 3nm, the thickness of the first GaN layer is 3nm, the thickness of the AlGaN insertion layer is 1~2nm, and the thickness of the second GaN layer is 3nm. In a preferred embodiment, the multi-quantum well layer includes a 5-period quantum well structure, the Al compositions of the AlGaN insertion layers 6, 7, 8, 9, and 10 are 0.04, 0.08, 0.12, 0.16, and 0.2, respectively, and the thickness of the AlGaN insertion layer is 2 nm.

[0034] The superlattice electron blocking layer consists of m periods of AlGaN / GaN stacks and an AlGaN thick layer disposed on the stacks, 2≤m≤5. In the AlGaN / GaN stacks, the thickness of the single AlGaN layer is 1-2nm, the thickness of the GaN layer is 1nm, and the doping type of the GaN layer is p-type with a doping concentration of 3×10 17 cm -3 The thickness of the AlGaN thick layer is 3 to 6 nm, its doping type is p-type, and the doping concentration is 3×10 17 cm -3 In a preferred embodiment, m is selected to be 5, and in the AlGaN / GaN stack, the Al composition of the AlGaN layer is constant, selected to be 0.2, and the Al composition of the AlGaN thick layer is 0.2.

[0035] In a multi-quantum well structure, the GaN and InGaN layers together provide space for carrier recombination. The variation in their energy bands significantly affects the efficiency of carrier recombination. Because electron mobility is much greater than hole mobility, most electrons escape from the potential wells without recombination. The present invention employs an AlGaN insertion layer to provide a gradually increasing electron barrier height, limiting the high-mobility electrons to remain within the potential wells as much as possible, giving them more opportunities to recombine with holes. This significantly improves carrier recombination efficiency and further shortens carrier lifetimes. This further enhances the optoelectronic and communication performance of the green LED device. The AlGaN insertion layer employs a gradient Al composition to achieve more uniform carrier concentration within the multi-quantum wells, improving current congestion, which is particularly pronounced in small-sized MINI LEDs. Furthermore, the AlGaN / GaN superlattice electron blocking layer provides a lower hole barrier height, facilitating faster injection of low-mobility holes into the potential wells and improving carrier recombination efficiency.

[0036] The P-type layer 14 is a p-type GaN layer doped with Mg, and its doping concentration is 3×10 17 cm -3 The n-type GaN layer is provided with an n-type electrode 16. Both the p-type electrode 15 and the n-type electrode 16 are transparent electrodes, and can be made of ITO, conductive polymer PEDOT, metal mesh, carbon nanorods, nanosilver wires, or graphene.

[0037] Figure 2 This is a schematic diagram of the energy band between the last QB and EBL of a green LED according to a preferred embodiment of the present invention. Considering the sidewall defects of the MINI LED, the electron trap energy level is 0.24eV (E c -0.24eV), the trap density and capture interface are 1×10 13 cm -3 and 3.4×10 -17 cm 2 The hole trap energy level is 0.46 eV (Ev + 0.46 eV), and the trap density and capture interface are 1.6 × 10 13 cm -3 and 2.1×10 -15 cm 2 Then we get the energy band between the last QB and EBL as Figure 2 As shown, the electron barrier height between the last quantum barrier and the EBL in the structure of the present invention is the highest, reaching 462.4 meV, effectively preventing electron leakage. Furthermore, the hole barrier of the LED is reduced, demonstrating that the addition of an AlGaN / GaN superlattice electron blocking layer can achieve higher hole injection.

[0038] Figure 3 This diagram illustrates carrier movement between the n-GaN layer, AlGaN / GaN multi-quantum well layer, SL EBL layer, and p-GaN layer of a green LED in a preferred embodiment of the present invention. As shown, the multi-quantum well and superlattice both have a period of five. When a forward operating current is injected into the LED device, electrons begin migrating from the n-GaN side. Some electrons are captured by the first potential well, while others escape the first well and migrate to the second, third, and subsequent wells. The gradually rising potential barrier slows electron migration, allowing electrons to be more evenly captured by multiple wells. On the other side, holes migrating from the p-GaN layer encounter a lowering potential barrier, accelerating their injection rate. Consequently, the captured electrons combine with holes injected into the p-side well.

[0039] Regarding the light output power, internal quantum efficiency, and modulation bandwidth of the green LED of the preferred embodiment of the present invention, the green LED of the present invention has obvious advantages over the other three conventional LED structures in terms of light output power, internal quantum efficiency, and modulation bandwidth. It makes the distribution of carriers more uniform, improves the radiation recombination rate of carriers in the device, reduces the carrier radiation recombination lifetime, and improves the light output efficiency and modulation bandwidth of the chip. In addition, electron leakage is reduced and hole injection is increased, and the current crowding effect is effectively alleviated, reducing the Droop effect under high current. The structural diagram of the active area of ​​the other three conventional LED structures LED A, LED B, LED C and the green LED of the embodiment of the present invention is shown in FIG. Figure 4-7 As shown, the structures of other layers are the same:

[0040] like Figure 4 As shown: LED A: Its MQW layer is an InGaN / GaN layer with 5 periods, the thickness of the InGaN layer is 3nm, and the thickness of the GaN layer is 3nm.

[0041] like Figure 5 As shown: LED B: Its MQW layer is based on the MQW layer of LED A, and an AlGaN barrier layer with a thickness of 1 to 2 nm and a fixed Al composition is inserted into the GaN layer, and the Al composition is 0.04 to 0.2.

[0042] like Figure 6 As shown: LED C: Its MQW layer is based on the MQW layer of LED A, and an AlGaN barrier layer with a thickness of 1 to 2 nm and gradually increasing Al composition is inserted into the GaN layer. The Al composition is 0.04, 0.08, 0.12, 0.16, and 0.2 respectively.

[0043] like Figure 7LED D is the structure used in this embodiment. Based on LED C, a superlattice electron blocking layer consisting of an AlGaN / GaN layer with a fixed Al content and a p-AlGaN thick layer is added. The thickness of the p-AlGaN thick layer is 3-6 nm, the thickness of the AlGaN layer is 1-2 nm, the thickness of the GaN layer is 1 nm, and the p-type doping concentration is 3×10 17 cm -3 . .

[0044] The device size of the four LED structures is 30um×30um.

[0045] Figures 8-10 Schematic diagram of the internal quantum efficiency, light output power and transient response of the four MINI green LED-D and LED-A, LED-B, and LED-C mentioned above.

[0046] The schematic diagram of the light output power of the MINI green LED-D and LED-A, LED-B, and LED-C under different currents in the embodiment of the present invention is shown in FIG. Figure 8 As shown in the figure, LED A has a light output power of only 47mW at an operating current of 200mA. LED B, based on LED A, inserts an AlGaN barrier layer with a constant Al composition and a thickness of 1-2nm. Lattice matching improves crystal quality, thereby reducing the electrostatic field and increasing the wave function overlap of electrons and holes, which increases the light output power to 70mW at the same current. LED C, based on LED A, inserts an AlGaN barrier layer with a gradually increasing Al composition and a thickness of 1-2nm. The gradient barrier layer has a more uniform distribution of electrons and holes, reducing non-radiative recombination, which further increases the light output power to 179mW at the same current. LED D, based on LED C, adds a superlattice electron blocking layer consisting of an AlGaN / GaN layer with a fixed Al composition and a p-AlGaN layer. This provides a lower hole barrier height, facilitates faster injection of low-mobility holes into the potential well, improves carrier recombination efficiency, and finally increases the light output power to 224mW.

[0047] Regarding the internal quantum efficiency of the MINI green LED of the preferred embodiment, Figure 9This is a schematic diagram of the internal quantum efficiency of the MINI green LED structure of the present invention and three other structures at different currents. The figure shows that the internal quantum efficiency of LED A and LED B is 0.6 and 0.7, respectively, while the internal quantum efficiency of LED C and LED D is as high as 0.9. The InGaN / GaN multi-quantum well layer, inserted with an AlGaN layer with a graded Al composition, gradually raises the potential barrier in the active region, making the carrier distribution more uniform and improving the internal quantum efficiency of the device. Furthermore, it can be seen that the droop effect of LED A, LED B, LED C, and LED D at a high current of 200mA decreases by 86%, 79%, 58%, and 39%, respectively. Among them, LED D has the least impact of the droop effect at high currents. This is mainly due to reduced electron leakage and increased hole injection, which effectively alleviates current crowding and reduces the droop effect at high currents.

[0048] Regarding the communication performance of the MINI green LED in the embodiment, Figure 10 This is a transient response curve of the light output power of a preferred embodiment of the present invention's green LED structure and the three comparative structures described above under a 60mA square wave current pulse. It can be seen from the figure that the rise time 1 of LED D is 6.38ns, the rise time 2 is 6.40ns, and the average rise time τ is 6.39ns. According to the bandwidth formula of the electro-optical-electrical modulation, The modulation bandwidth of this structure is 93.8 MHz. The modulation bandwidths of the other three structures, LED A, LED B, and LED C, are 76.9 MHz, 75.4 MHz, and 91.5 MHz, respectively. It can be seen that the modulation bandwidth of the structure used in this embodiment is higher than that of the other three structures. This is mainly because there are more recombination carriers inside the active region, and the injected carrier concentration is greater, resulting in a greater recombination probability, a shorter carrier lifetime, and a correspondingly higher modulation bandwidth, making it more suitable for communication system applications.

[0049] In optical communication applications, half-power bandwidth (or 3dB bandwidth) is of great significance for the frequency selectivity, signal integrity and performance evaluation of the system. According to the theoretical bandwidth formula of electro-optical-electrical modulation As can be seen, where π is pi, q is the basic electron charge, d is the thickness of the active layer, η is the carrier concentration of IQE under steady-state conditions, A is the Schottky recombination coefficient, B is the radiative recombination coefficient, and C is the Auger recombination coefficient. It can be seen that the increase in recombined carriers within the active region, accompanied by an increase in the probability of radiative recombination of carriers, shortens the radiative recombination lifetime, and correspondingly increases the 3dB modulation bandwidth of the LED. This is consistent with the aforementioned increase in modulation bandwidth from 76.9MHz to 93.8MHz at a turn-on voltage of 3.86V and 60mA.

[0050] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A green LED, characterized in that: It includes an n-type layer, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer and a p-type electrode stacked in sequence on a substrate; The multi-quantum well layer includes a quantum well structure of n periods, wherein a single period of the quantum well structure is formed by sequentially stacking an InGaN layer, a first GaN layer, an AlGaN insertion layer, and a second GaN layer. In the multi-quantum well layer, the Al component of the AlGaN insertion layer increases layer by layer in a direction from the substrate to the p-type layer, and 2≤n≤5; The superlattice electron blocking layer is composed of m periods of AlGaN / GaN stacking layers and an AlGaN thick layer provided on the stacking layers, 2≤m≤5, and in the AlGaN / GaN stacking layers, the Al composition of the AlGaN layer is 0.2; In the multi-quantum well layer, the Al composition of the AlGaN insertion layer in the quantum well structure of the first cycle is 0.04, the Al composition of the AlGaN insertion layer in the quantum well structure of the last cycle is 0.2, and the difference in the Al composition of the AlGaN insertion layer in the quantum well structure of adjacent cycles is constant; In a single-period quantum well structure, the thickness of the InGaN layer is 3nm, the thickness of the first GaN layer is 3nm, the thickness of the AlGaN insertion layer is 1~2nm, and the thickness of the second GaN layer is 3nm.

2. The green LED according to claim 1, characterized in that In the AlGaN / GaN stack, the thickness of the AlGaN layer is 1-2 nm, and the thickness of the GaN layer is 1 nm.

3. The green LED according to claim 1 or 2, characterized in that: The thickness of the AlGaN thick layer is 3-6 nm, and the Al composition of the AlGaN thick layer is 0.

2.

4. The green LED according to claim 3, characterized in that The p-type layer is a p-type GaN layer.

5. The green LED according to claim 3, characterized in that: The n-type layer is an n-type GaN layer, and an n-type electrode is further provided on the n-type GaN layer.

6. The green LED according to claim 3, characterized in that The substrate is a sapphire or silicon carbide substrate.

7. The green LED according to any one of claims 1, 2, 4, 5 and 6, wherein: The green LED is a MINI green LED.

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